Attackers could take over the router and execute commands simply by sending a UDP packet with a special string, as explained in a detailed advisory.

"The backdoor only listens on the LAN, thus it is not exploitable from the WAN," Heffner explains. "However, it is exploitable over the wireless network, which has WPS enabled by default with no brute force rate limiting. My shiny new ReaverPro box made relatively short work of cracking WPS, providing access to the WLAN and a subsequent root shell on the router.'

Heffner warned that the backdoor was present on Tenda’s W302R and W330R router models as as well as re-branded models, such as the Medialink MWN-WAPR150N, because all share the same exploitable functionality (specifically a “w302r_mfg” magic packet string in a modified GoAhead webserver) that ships with the products.

Attackers could gain unauthenticated access to the routers’ administrative interfaces through its built-in web server on all affected models, provided they had first managed to hop onto the same network as their intended victim.

Target Components, which distributes Tenda's products in the UK, acknowledged the problem while crediting Tenda for developing a fix in just two days.

The company had released a software update that closes the backdoor on the three affected models. Target added that, contrary to fears expressed by other security researchers in they wake of Heffner's discovery, the backdoor problem was restricted to three models of Tenda-manufactured kit.

In a statement, Tenda apologised to its customers and said: “We promise that no other lines of Tenda have the same bugs after all lines were detected systematically by our engineers.”

It was also verified by our engineers that the bug will not make your network system be visited via internet by strangers, so we can promise that there is no big security problem occurred. However, we would suggest that all the SME users who use those two lines upgrade them immediately just in case of further security problems.

The relevant security patches can be found via links within Tenda's advisory here.

We passed on Tenda's advisory to Heffner but are yet to hear from him. Thus there's no confirmation that the fix released by Tenda actually does what it says on the tin.

Earlier this month Heffner discovered a similar backdoor on a range of router kit from D-Link. In response, D-Link promised to close its routers' backdoors by Halloween (31 October).

Pending the availability of a fix, users of the vulnerable kit are advised to disable remote access to their routers and make sure their wireless networks are secure. ®